Trolling from a boat is a favorite and very effective way to catch most freshwater and saltwater fish. The troll depth of the lure or bait is important in finding and catching fish but it is usually not known.
For most conventional trolling an angler ties on his or her favorite bait or lure, and maybe trolling tackle like weights and fish attractors, and lets out the length of line that they think is appropriate. The line makes a shallow angle behind the boat and anglers usually think the lure or bait is shallow. The line makes an arc due to the downward pull of the weight, however, and thus extends deeper than it appears. Misapprehension of trolling depth often causes lures to miss targeted fish populations or snag the bottom.
Anglers have estimated line depth using basic trigonometry and assuming the line maintains the same angle from the lure or bait to the surface. This practice leads to considerable error for lines with appended weights, however, because the curvature of such a line increases as it approaches the distal end.
The inventor wrote and solved complex equations that produce accurate troll-depth estimations. These efforts are outlined in Ray Rychnovsky, “The Troller's Handbook for All American Fish Species” (1998). Mr. Rychnovsky details how four major factors influence line curvature, and thus troll depth. These factors are line diameter, the weight appended at or near the end of the line, trolling speed, and the length of the line below the water surface. Of these factors, trolling speed and the appended weight have the largest effects on troll depth. Recognizing the difficulty of graphing four factors, the Troller's Handbook provides charts and tables to aid the angler in estimating troll depth. As one can imagine, however, it can be cumbersome to measure and juggle the major factors that influence line curvature, and thus to maintain an accurate sense of troll depth.